Platinum clusters

Trevor D J, Whetten R L, Cox D M and Kaldor A 1985 Gas-phase platinum cluster reactions with benzene and several hexanes evidence of extensive dehydrogenation and size-dependent chemisorption J. Am. Chem. Soc. 107 518... 

Quantum mechanical calculations electrochemical promotion, 267 with copper clusters, 268 with platinum clusters, 268 Quasi-reference electrodes, 118... 

Parmigiani F, Kay E, Bagus PS. 1990. Anomalous oxidation of platinum clusters studied by X-ray photoelectron-spectroscopy. J Electron Spectrosc Relat Phenom 50 39-46. 

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

Kineti cs. To date only addition reactions have been reported. These reactions produce products or adducts that are the result of complete addition, or addition and subsequent elimination. An example of the later reaction is dehydrogenation of hydrocarbons on platinum clusters. These addition reactions are in many ways analogs to the chemisorption process on metal surfaces. 

Platinum clusters, n = 2-11 react with di oxygen at a rate that is within an order of magnitude of gas kinetic. There is no distinct size selective behavior. Products of these gas phase reactions observed with 7.87 eV ionization laser, are PtpO where for m=l,... 

Other Cg hydrocarbons. The dehydrogenation of normal hexane and 2,3-di methyl butane also proceeds but not as voraciously on small platinum clusters. Figure 8 is a plot of the hydrogen content in the first adduct as a function of the size of the platinum metal cluster. The metal atom reacts via dihydrogen elimination to produce PtC6Hi2 products. The platinum trimer is now the smallest cluster that will produce a C H near one. The similarity of size dependent dehydrogenation of the normal hexane and the branched molecule suggest that these systems may not readily aromatize these alkanes. Further structural studies are needed to identify the reaction products. 

Figure 7. A plot of the adduct masses (product - bare metal) produced in reacting platinum clusters with c-CgDjo. The lines for the first adduct (solid) and second-first (dashed) adduct masses are drawn to help guide the eye through the scatter in the data.

Figure 8. Plots of the number of hydrogens retained in the products formed in reacting platinum clusters with n-CgHia and...

In summary a few "generalizations" have been found. First, size selective chemistry is strongly associated with chemisorption that requi res bond-breaking. Second, metal clusters react rapidly with ligands that molecularly chemisorb even when the eventual products involve dissociation of the ligand. Dehydrogenation of Cg-alkanes on small platinum clusters take exception to this. 

Electrospray ionization will often produce ions that are fully coordinated, stable, and nonreactive in the gas phase. These ions may be probed by removal of ligands to form coordinatively unsaturated ions that are generally reactive. The chemical activity of metal cluster ions differs markedly and often shows size specific enhanced reactivity or lack of reactivity. Silver cluster ions Ag are fairly inert similar to Ag+. Platinum cluster ions PL are quite reactive similar to Pt+. Often, large cluster ions only appear to react with one donor molecule such as benzene this may be due to low concentrations of reactants or short reaction times. Similar clusters may react with a larger number of smaller molecules, and so until more information is available, rules for the coordination behavior of metal clusters are as yet not available. 

Toshima, N., Takahashi, T., and Hirai, H., Polymerized micelle-protected platinum clusters-preparation and application to catalyst for visible light-induced hydrogen generation, J. Macromol. Sci. -Chem., A25, 669,1988. 

Hsieh, C.-T., et al., Fabrication of flower-like platinum clusters onto graphene sheets by pulse electrochemical deposition. Electrochimica Acta, 2012. 64(0) p. 177-182. 

Another approach has been developed to fabricate electrodes with loading as low as 0.1 mg Pt/cm (32). The electrode structure was improved by increasing the contact area between the electrolyte and the platinum clusters. The advantages of this approach are that a thinner catalyst layer of 2 to 3 microns and a uniform mix of catalyst and ionomer are produced. For example, a cell with a Pt loading of 0.17 to 0.13 mg/cm has been fabricated. The cell generated 3 A/cm at > 0.4V on pressurized O2 and 0.65 V at 1 A/cm on pressurized air (32,... 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

Molybdenum nitride itself has been dispersed on platinum clusters dispersed in EMT zeolite. In this study, coverage of platinum particles by molybdenum deposition was investigated and was reported to suppress the activity for benzene hydrogenation and enhance the heptane conversion. With respect to bare Pt for heptane conversion, the aromatisation activity was reduced, whereas isomerisation and hydrogenolysis were observed. 

Fig. 28. Schematic representation of a platinum catalyst with a monolayer of carbonaceous overlayer showing the exposed platinum clusters.

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